Toward the Realization of a 21-GHz-Band Satellite Broadcasting System There is great anticipation that satellite broadcasting using the 21-GHz frequency band will develop into a transmission medium providing new broadcasting services, such as ultrahigh-definition TV and 3D TV services, in addition to multi-channel HDTV broadcasting, based on broadband characteristics. On the other hand, radio signals are prone to suffer from greater rain attenuation at higher frequencies, necessitating technical ingenuity to overcome such rain attenuation in order to realize this 21-GHz-band satellite broadcasting system. 1. Introduction four additional channels (channels 17, 19, 21, and 23) that Satellite broadcasting on the 12-GHz-band, which began were assigned at the WRC-2000 (a World regular services in 1989, reached its 15th anniversary in Radiocommunication Conference held in 2000). It is, June 2004. It started with a total of three analog channels, however, important to consider advanced satellite consisting of two NHK channels (which began in 1989) broadcasting applications that will utilize the broader and one channel by WOWOW (which began in 1991). band of the 21-GHz-band, or even the millimeter-wave This was followed by the opening of digital satellite bands, in view of transmitting ultrahigh-definition TV broadcasting, which added an additional six TV (Super-HiVision) with 4,000 scanning lines and 3D TV broadcasters, sound service broadcasters, and data service services (Figure 1), which are anticipated new future broadcasters. It has grown into an indispensable broadcasting services, in addition to multi-channel HDTV broadcasting medium for the daily lives of people in programming. Japan. While one of the main purposes of this satellite Radio signals have the property of being more prone to broadcasting was to act as a fundamental countermeasure attenuation caused by rainfall (rain attenuation) at higher to eliminate the TV viewing difficulties that still remained frequencies. This fact raises the need for satellite in some of the mountainous areas and remote islands of broadcasting utilizing the high frequency bands to be Japan, it has come to serve a role as a broadcasting prepared with countermeasures for this problem. medium responding to a diverse range of demands by viewers. 2. Mitigation Techniques for Rain Attenuation Examinations are also being made with regard to the Service interruption by severe rainfall is an unavoidable application of new channels, including channels that will phenomenon in satellite broadcasting. This is caused by become available when the analog HDTV service channel rain attenuation, in which the rain absorbs or disperses the are terminated in 2007, and when all analog satellite broadcast-waves, resulting in a signal level that is lower broadcasting is to be terminated in 2011, together with the than a necessary level for reception. For digital satellite Ultrahigh-definition TV (Super HiVision) 3D TV Figure 1: Video system with high sensation of reality 6 Broadcast Technology no.20, Autumn 2004 C NHK STRL Feature Table 1: Rain attenuation countermeasure examples Category System Features, etc. (a) Uniform increase of In addition to the fact that it is not an efficient compensation transmitting power method due to the fact that uniform rain attenuation does not across service area occur nationwide, satellite transmitting power restricts service availability rate improvements. It may exceed the power-flux 1 Design enhancements density (pfd) limit in areas with fine weather. for on-board satellite transponder (b) Selective increase of Although this is an efficient compensation system that transmitting power only compensates by increasing transmitting power only in the areas over the area with with rain attenuation, it requires the constant monitoring of rain attenuation rainfall status to control satellite transmitting power accordingly. (c) Adoption of a scheme This is a system, which has been adopted in digital satellite with reception capability broadcasting, that adaptively adjusts a modulation scheme and at a low CNR an error correction scheme. Its transmission capacity decreases as (hierarchical transmission) a modulation scheme which is poper for receiving lower CNR signals is adopted. (d) Long-block length This system transmits dispersed transmitting data over a long interleave period, to recover the original data through error correction based 2 Transmission scheme on such dispersedly received data blocks, even when the reception ingenuities of data is interrupted at one point. It cannot be employed when data must be broadcast in real-time. (e) Time diversity By transmitting the same program several times in a certain interval, this system recovers the original data by replacing the portion with reception interruption with that from a correctly received session at a receiver. The transmission capacity decreases in inverse proportion to the number of times a program is re-transmitted. (f) Simultaneous transmission With the application of the characteristic that severe rainfall from multiple satellites occurs locally, this system is capable of receiving broadcast-waves 3 Others located at separate orbits from multiple satellites with different arrival directions, to select (satellite diversity) the most favorable signal at the time of rainfall. It does not necessarily provide effective use of satellite orbits. broadcasting utilizing the 12-GHz-band, it is estimated that (1) and method (d) in category (2) is being carried out in this service interruption will amount to approximately two the Science & Technical Research Laboratories. hours in the worst month (the month with the most rainfall), or approximately eight hours in a year, in the 3. Approaches Being Made by the Science & case of receiving the broadcast using the typical type of Technical Research Laboratories receiver with a 45 cm-diameter antenna in Tokyo. The Current examinations on mitigation techniques for rain effect from rainfall is thought to be more significant as attenuation by the Science & Technical Research higher radio frequencies are employed. For instance, the Laboratories principally involve a variable radiation- degree of rain attenuation for a 21-GHz-band broadcast- pattern satellite system and its related element wave is estimated to be approximately three times higher technologies, as well as a long-block length interleave than that of a broadcast wave at the 12-GHz-band in dB transmission scheme. This section provides overviews of unit, so it is thought that even a relatively minor rainfall these research projects. could broadcast. Therefore, the most important subject toward realizing satellite broadcasting over the 21-GHz- 3.1 Variable radiation-pattern satellite system band is to reduce the influence of this rain attenuation, overview1 and reduce service interruptions as much as possible. The higher the frequency used for radio waves, the more The countermeasures for rain attenuation can be significant rain attenuation becomes. This makes a roughly categorized into the following three aspects, (1) method for overcoming rain attenuation the key to design enhancements for the on-board satellite satellite broadcasting utilizing a higher frequency band, transponder, (2) transmission system ingenuity, and (3) such as the 21-GHz band. As mentioned earlier, rain others. More specifically speaking, system enhancements attenuation in the 21-GHz-band reaches approximately such as described in (a) to (f) in the inserted Table 1 can be three times in dB unit that of the 12-GHz-band, so considered. Upon a consideration of the technical and mitigation techniques for rain attenuation must be applied service potential in these methods, research and for reduction of service interruption. It has been observed development that extends to related technologies, that rainfall accompanies regional and temporal especially noting method (b) related to the above category fluctuations, and that severe rain, causing significant rain Broadcast Technology no.20, Autumn 2004 C NHK STRL 7 Large-scale reflector 3.2 Rainfall and rain attenuation analysis and satellite Large phased-array antenna system design The measurement and analysis of the rainfall occurring over Japan, and the radio wave propagation Array characteristics observed during such rainfall, are essential element to migration of rain attenuation compensation with the boosted beam formed using a variable radiation-pattern satellite system. Ideally, fluctuations in the strength of Boosted beam received broadcast-waves should be gauged at satellite for rainfall areas signal reception monitoring stations established nationwide, to feed back the results to the satellite transponder. Such an expensive construction of monitoring stations is, however, difficult due to financial Minimum power required for broadcasting serservicevice constraint. Also released rain attenuation data measured so far is extremely limited in some locations. For this reason, future studies must progress with the use Naha Tokyo Osaka of data from the Radar AMeDAS, on methods such as Sendai Nagoya Sapporo Fukuoka estimation of areas that will require a boosted beam and Hiroshima Radiation power from satellite the boosting amount in advance. Also to use Rader Figure 2: Variable radiation-pattern satellite system concept for AMeDAS weather forecasting it is need to understand the rain attenuation mitigation geographical and temporal relation between rainfall and rain attenuation. We are studying a method to